1. Field of the Invention
The present invention is related to the field of filter elements and to methods for manufacturing such filter elements. More specifically, the invention relates to such filter elements that can be regenerated using microwaves.
2. Description of Related Art
Exhaust gases from internal combustion engines and other industrial devices have been found to be detrimental to the environment and are considered to be major contributors to various environmental pollution problems including air pollution. Typically, incomplete combustion of petroleum-based fuels such as diesel fuel, causes emission of carbon containing particulates. Because of the detrimental effect on the environment, exhaust emissions of internal combustion engines and other industrial devices have been subjected to ever increasing scrutiny and regulation by governmental agencies which are mandating reduced particulates and other pollutants from exhaust emissions.
In response, various types of high temperature exhaust filtration devices have been proposed in the art for removing particulates from exhaust gases. In such filtration devices, when the filter element has collected a certain quantity of particulates, the pressure drop across the filtration device becomes excessive. Then, the filter element must be either replaced or regenerated. In this regard, by further increasing the temperature of the filtration element and the particulates, incineration of the particulates collected on the filter element may be attained so that the filtration device and the filter element may be returned into service.
Various regeneration techniques have been developed and applied especially in diesel engine exhaust engine applications. Because diesel particulates do not undergo significant oxidation at normal operating exhaust temperatures of less than approximately 400xc2x0 C., these regeneration techniques typically seek to increase the temperatures of the particulates to beyond 400xc2x0 C. to incinerate these particulates. For example, diesel engines"" exhaust temperatures can be raised to above 500xc2x0 C. to induce filter element regeneration by throttling the engine. However, this technique requires operator intervention and the suspension of normal engine operation for a period of time. Because of these disadvantages, throttling has not been widely adopted as a suitable method of filter element regeneration. In other techniques, alternative heat sources such as burners or resistance heaters are used to raise the exhaust temperature and/or filter temperature to regenerate the filter element. However, these techniques have also been shown to have limitations in that they are not very reliable in initiating particulate incineration and can lead to incomplete filter element regeneration. In certain instances, such techniques can even destroy the filter element by causing localized thermal stresses.
To avoid the disadvantages of these filter regeneration techniques, a filter regeneration technique of providing microwave energy to the filter element has been applied recently. For example, U.S. Pat. No. 5,087,272 to Nixdorf discloses a filtration apparatus which may be regenerated by heating a filter element via microwave energy. The reference discloses that the filter element is heated through numerous very small whiskers of discontinuous silicon carbide fibers substantially homogeneously distributed throughout the filter element. Similar to the foregoing, U.S. Pat. No. 5,074,112 to Walton et al. discloses a filter assembly for filtering the exhaust gases of a diesel engine including a mechanism from providing microwaves to regenerate the filter element. In this regard, in a similar manner to the Nixdorf reference, Walton teaches that the filter element is evenly coated with a ferrite susceptor material which absorbs microwave energy and produces the heat required to incinerate the trapped particulates of the filter element.
In application, however, the above noted filter regeneration devices and filter elements as described in Nixdorf and Walton references have been found by the present applicants to be inadequate for fully regenerating the filter elements. In particular, it has been found that such filter elements having a substantially homogenous and evenly distributed whiskers or coating of microwave absorbing materials do not allow even regeneration of the filter element, particularly in the portion of the filter element which are furthest distance from the microwave source. The present inventors believe this uneven heating and regeneration results because most of the microwave energy provided to the filter element is absorbed by the microwave absorbing material closest to the microwave source. Consequently, it is believed that the portion of the filter element furthest from the microwave source does not receive sufficient microwave energy to heat to a temperature which will incinerate the particulates in the central portion of the filter. Regardless of the cause, the filter elements as disclosed in the prior art have been found to be inadequate in providing an optimal filter element which can be effectively and fully regenerated using microwave energy.
Therefore, there exists an unfulfilled need for a filter element that can be regenerated using microwave energy while avoiding the limitations of prior art filter elements. In this regard, there exists an unfulfilled need for such a filter element which will provide even heat generation from the location where microwave energy enters the filter to the furthest regions away from this location. There further exists an unfulfilled need for a method for providing such a filter element in an economical manner.
In view of the foregoing, it is an object of the present invention to provide an improved filter element that can be regenerated using microwave energy.
A second object of the present invention is to provide such a filter element which will provide tailorable heat generation from the microwave energy.
A third object of the present invention is to provide such a filter element which can be designed to provide even heat generation or some heterogeneous heat generation distribution, even at portions of the filter element which are furthest from the microwave source to thereby allow full regeneration of the filter element.
Yet another object of the present invention is to provide a method for manufacturing such a filter element in an efficient and economical manner.
In accordance with preferred embodiments of the present invention, these objects are obtained by an improved microwave regenerable filter element including a filter media for filtering particulates from an exhaust gas, and a plurality of discrete heat generators disposed in the filter media, each of the plurality of discrete heat generators being adapted to absorb microwave energy and convert the microwave energy into heat. In accordance with one embodiment of the present invention, the filter media includes a refractory filter sheet which absorbs substantially less microwave energy than the plurality of discrete heat generators. The filter sheet is preferably spirally wound to form a plurality of wound filter layers and the plurality of discrete heat generators are disposed between the plurality of wound filter layers in a spatial array. The filter media may also include a corrugation sheet which is spirally wound together with the filter sheet to form a plurality of wound corrugation layers disposed between the plurality of wound filter layers. In one embodiment, the filter sheet and the corrugation sheet may be wound around a centrally positioned mandrel. The refractory filter sheet and the corrugation sheet may be made of ceramic fibers. In this regard, the ceramic fibers may be at least one of alumina fibers and alumina silicate fibers and the plurality of discrete heat generators may be provided on the refractory filter sheet and/or the corrugation sheet by at least one of laminating, embedding and printing.
In accordance with another embodiment of the present invention, the filter media of the filter element may be a monolithic refractory filter made of a ceramic fiber which is substantially transparent to microwave energy and include a plurality of discrete heat generators disposed in the monolithic refractory filter. The ceramic fiber may be at least one of alumina and alumina silicate and the plurality of discrete heat generators are preferably disposed in the monolithic filter material in a spatial array.
In accordance with one embodiment of the present invention, the plurality of discrete heat generators may be substantially planar shaped having a length dimension and a width dimension which are substantially larger than a thickness dimension. The plurality of discrete heat generators may be shaped as at least one of a polygon, a circle and an ellipse. In this regard, each of the plurality of discrete heat generators may have a length dimension of substantially xc2xd of a microwave wavelength such as approximately 6.1 centimeters. In addition, each of said plurality of discrete heat generators may have a width dimension of substantially xe2x85x9 of a microwave wavelength such as approximately 1.5 centimeters. Each of the plurality of discrete heat generators may also be made of a plurality of microwave absorption fibers attached together to form each of the plurality of discrete heat generators.
In accordance with yet another embodiment of the present invention, the plurality of discrete heat generators may be disposed in a spatial array in a manner that density of the plurality of discrete heat generators varies radially away from a central axis of the filter element. In particular, the density of the plurality of discrete heat generators in the spatial array may be increased radially away from the central axis of the filter element. Alternatively, the density of the plurality of discrete heat generators in the spatial array may be decreased radially away from the central axis of the filter element. Moreover, in accordance with another embodiment, the density of the plurality of discrete heat generators in the spatial array may vary axially along the central axis of the filter element.
In accordance with another aspect of the present invention, these objects are obtained by an improved method of manufacturing a microwave regenerable filter element including the steps of providing a filter media for filtering particulates from an exhaust gas, and providing a plurality of discrete heat generators disposed in the filter media in a spatial array, each of the plurality of discrete heat generators being adapted to absorb microwave energy and convert the microwave energy into heat.
In one embodiment of the present invention, the present method may further include the step of spirally winding a filter sheet to form the filter media having a plurality of wound filter layers. In addition, the present method may further include the step of spirally winding a corrugation sheet together with the filter sheet to form a plurality of wound corrugation layers disposed between the plurality of wound filter layers. In accordance with the present invention, the step of providing the plurality of discrete heat generators may be attained by at least one of laminating, embedding and printing the plurality of discrete heat generators on the filter sheet and/or corrugation sheet.
In another embodiment of the present invention, the present method may also include the step of providing each of the plurality of discrete heat generators in a predetermined manner such that density of the plurality of discrete heat generators in the spatial array varies radially away from a central axis of the filter element. In this regard, each of the plurality of discrete heat generators may be provided in a predetermined manner such that density of the plurality of discrete heat generators in the spatial array increases radially away from the central axis of the filter element. Alternatively, each of the plurality of discrete heat generators may be provided in a predetermined manner such that the density of the plurality of discrete heat generators in the spatial array decreases radially away from the central axis of the filter element. Moreover, each of the plurality of discrete heat generators may be provided in a predetermined manner such that the density of the plurality of discrete heat generators in the spatial array also varies axially along the central axis of the filter element.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when viewed in conjunction with the accompanying drawings.